1. Field of the Invention
This invention relates to an apparatus which controls the running direction of a trackless moving body adapted to make running movement along a guide wire so that it can be accurately guided to run along the guide wire.
2. Description of the Prior Art
A transfer crane supported to run with rubber tyres as shown in FIG. 1 is known as an example of a trackless moving body. This transfer crane is used as a cargo-handling machine which hoists and carries a container in a container yard or terminal.
The crane of this kind includes a trolley 2 capable of making traversing movement on a pair of girders 1, and a spreader 3 vertically movably suspended from the trolley 2 for releasably grasping a container 4. Rubber tyres 5, 6, 7 and 8 are mounted on the lower ends of the four legs respectively of the transfer crane shown in FIG. 1. These tyres 5 to 8 are arranged at four relative positions as shown in FIG. 2. The rubber tyre 5 is driven for rotation by a motor 9, and the rubber tyre 8 is driven for rotation by another motor 10. The remaining rubber tyres 6 and 7 are idlers with no connection to such driving motors.
The transfer crane is arranged to run along a straight path when the two motors 9 and 10 are simultaneously driven at the same rotation speed. The running direction of the transfer crane can be changed so that it can run leftward or rightward from the straight path when one of the two motors 9 and 10 is deenergized or when the rotation speed of one of the two motors 9 and 10 is made higher or lower than that of the other. The transfer crane can thus be steered to run in any desired direction.
Practically, however, the difference between the diameters of the individual rubber tyres 5, 6, 7, 8, the difference between the running resistances and loads imparted to the individual rubber tyres 5, 6, 7, 8 and the unbalance of the operating characteristics of the individual motors 9, 10 have made difficult to run the crane along a straight path in spite of rotation of the two motors 9 and 10 at the same rotation speed. This tendency becomes more marked with the increase in the running speed of the crane. Further, when a crane operator sits in an operator's box 11 annexed to the trolley 2 to control the running movement of the crane, he must also control the traversing movement of the trolley 2 and vertical movement of the spreader 3, and his burden is quite heavy in both the physical aspect and the mental aspect. A skilled operator has therefore been required for the successful control of the crane operation.
However, it has become more and more difficult to obtain such a skilled crane operator in recent years, and it has been strongly demanded to furnish the crane of this kind with a function of automatic steering control so that the crane can be automatically guided to run along a straight path or track.
In Japanese Utility Model Publication No. 53-39007 published on Sept. 21, 1978, there is disclosed a control apparatus which is applicable to the crane of the kind above described so that the crane can be automatically controlled to run along a straight path.
In the above control apparatus, as shown in FIG. 3, the two drive motors 9 and 10 are connected to a common generator 12, and the current supplied to one of the field windings 13 and 14 of the respective motors 9 and 10 is changed by short-circuiting a part of associated field resistor 13a or 14a through a relay contact 33a or 34a as mentioned hereinafter, thereby to attain the desired automatic steering control. For the purpose of running speed control, a voltage command signal is applied from a controller 15 to a control unit 16, and an output signal from the control unit 16 is applied to a thyristor 17 to vary the field current of the generator 12, so that the output voltage of the generator 12 can be varied in a stepless fashion thereby changing the rotation speed of the two motors 9 and 10. For the purpose of steering control, a guide wire 18 is previously buried in the ground surface GL, and a very weak current at a frequency of the order of several kHz is continuously supplied to this guide wire 18. Antennas 19 are mounted on the crane as a means for sensing this very weak current flowing through the guide wire 18. The sensor output signals pass through a detector circuit 20, a rectifier circuit 21 and a differential amplifier circuit 22 to appear as a position deviation output signal 23 which is applied to a steering control circuit 35. Thus, the antennas 19, detector circuit 20, rectifier circuit 21 and differential amplifier circuit 22 constitute a position deviation sensor unit 36. In the steering control circuit 35, this position deviation signal 23 is differentiated by the primary or the first order differentiation circuit 24 to obtain a primary differentiation signal 25, and this primary differentiation signal 25 is then differentiated by a secondary differentiation circuit 26 to obtain the secondary or the second order differentiation signal 27. Then, the position deviation signal 23, primary differentiation signal 25 and secondary differentiation signal 27 are applied to an adder 31 through respective signal amplifiers 28, 29 and 30. The output signal from the adder 31 is applied to a dead-zone comparator 32 connected to a pair of relays 33 and 34 for correcting any steering error, the relay 33 has a relay contact 33a for changing the field current of the motor 9, and the relay 34 has a relay contact 34a for changing the field current of the motor 10. Any one of the relays 33 and 34 is turned on and off to change the effective value of the field resistor thereby changing the field current of the motor 9 or 10 thereby correcting the steering error. In this manner, the running track of the transfer crane is corrected to follow the guide wire 18.
According to the principle of steering error correction above described, the control output S is given by EQU S=D-V.theta.-Vw
where D is the amount of crane position deviation, V is the running speed of the crane, .theta. is the angle of crane position deviation relative to the guide wire 18, and w is the angular velocity of the crane position deviation. The value of V.theta. is calculated by the primary differentiation of the amount D of crane position deviation, and the value of Vw is calculated by the secondary differentiation of V.theta.. However, due to the fact that the period of variation in the crane position deviation is practically quite long, and both the angle .theta. and the angular velocity w are very small as shown in FIG. 4, the amplifiers 29 and 30 connected to the outputs of the respective differentiation circuits 24 and 26 had to have a very large amplification degree in order to successfully find the values of V.theta. and Vw.
Further, in view of the fact that the differentiation circuits 24 and 26 tend to produce oscillations even in their steady state, the requirement for such an increase in the amplification degree of the amplifiers has resulted in a more complex circuit structure. Furthermore, a filter circuit for filtering the position deviation signal 23 has been required because direct application of this signal, which includes noise components of 1 to 10 Hz, to the differentiation circuit 24 results in appearance of an erroneous steering error correction signal at the output of the steering control circuit 35.